1. Field of the Invention
The present invention relates to a gaseous fuel injector, which directly injects a high-pressure gaseous fuel to cylinders of an internal combustion engine.
2. Description of Related Art
In development of a next generation vehicle, a clean exhaust gas and a decrease of carbon dioxide are greatly required. However, conventional combustion technology using liquid fossil fuel may not reach this requirement. Development of a gaseous fuel engine using gas fossil fuels and hydrogen gas as alternative fuels for the liquid fossil fuel is progressing and some of the gaseous fuel engines are actually used. Here, the gas fossil fuels, which expectedly contribute effective combustion, include a natural gas and a petroleum gas.
A cylinder direct injection engine may not be used long in a case where a conventional gaseous fuel is used, because the conventional gaseous fuel has low lubricity. Thus, there may be reliability disadvantages, such as wear and burn-in of a nozzle sliding portion of the injector, and sealing failure caused by the wear of a valve portion. Also a compression ignition engine may have a disadvantage, such as an unstable operation with the gaseous fuels (e.g., the hydrogen gas and the natural gas, which have low cetane numbers).
To improve reliability and operational performance, several techniques are proposed (see Unexamined Japanese Patent Publication No. 2003-232234 and Unexamined Japanese Utility Model Publications No. S63-4365 and No. H1-88054).
Unexamined Japanese Patent Publication No. 2003-232234 discloses a two-fuel injection technique, which supplies either or both of the gaseous fuel (e.g., LPG) and the liquid fuel (e.g., gasoline) from corresponding injectors based on a state of an engine operation. According to the technique, the first fuel supply system, which supplies the gaseous fuel from the first injector, and the second fuel supply system, which supplies the liquid fuel from the second injector, are provided. In a transition from an operational state using the liquid fuel to an operational state using only the gaseous fuel, the first and second injectors simultaneously supply the gaseous fuel and the liquid fuel to the internal combustion engine to avoid an excessive lean state of an air fuel ratio. Thus, misfire or deterioration of the operational performance is limited.
Unexamined Japanese Utility Model Publications No. S63-4365 discloses a gas injector for injecting gas supplied from an accumulator to lubricate the sliding portion of the needle by a seal oil, which is supplied to the injector through an additionally provided passage to seal the needle portion. The gas injector also includes an inlet passage for supplying a working fluid to open and close a valve portion. Unexamined Japanese Utility Model Publication No. H1-88054 discloses an injector that has an improved lubricity of the needle by using the working fluid as a lubrication and storing the working fluid and the gaseous fuel in a seal oil storage.
In practice, the technique of Unexamined Japanese Patent Publication No. 2003-232234 largely depends on the liquid fuel to operate the engine in order to limit an evaporation of the liquid gaseous fuel. Thus, an effect of using the gaseous fuel may not be maximized. Also, an apparatus of the invention may become large, because the technique requires two large fuel tanks for the gas liquid and the liquid fuel, and also requires two fuel supply systems and two fuel injector valves for the two fuel tanks.
In the invention according to Unexamined Japanese Utility Model Publication No. S63-4365, the composition of the technique may be complex, because the seal oil, as well as the working fluid that drives the injector, is also supplied to the injector. Moreover, the seal oil may continuously leak into the working fluid and the gaseous fuel, because a pressure of the seal oil is applied to a seal groove of the needle. In the invention according to Unexamined Japanese Utility Model Publication No. H1-88054, a lower portion of the needle is not lubricated sufficiently, because the working fluid does not easily reach a lower portion (or an injection hole side) of the seal oil storage due to pressure of the gaseous fuel, which is applied therein. Also, the inventions according to Unexamined Japanese Utility Model Publications No. S63-4365 and No. H1-88054 are not effective for decreasing wear of the valve portion, because both inventions improve the lubricity of only the sliding portion of the needle.